1. Field of the Invention
The present invention relates to a focus adjusting apparatus that can obtain a video signal that can be processed in an imaging system, such as a video camera, and relates to a focus adjusting method.
2. Description of the Related Art
An automatic focus adjustment (AF) apparatus can be employed for a video device, such as a video camera. For example, a conventionally known TV-AF system extracts a high frequency component, as a focus signal, from a video signal obtained by an image-capturing unit, such as a complementary metal oxide semiconductor (CMOS), and performs focus adjustment by driving a focus lens in such a way as to increase the magnitude of the focus signal. The magnitude of the focus signal can be expressed using an “AF evaluation value.”
As discussed in Japanese Patent Application Laid-Open No. 7-298120, an AF evaluation value for the TV-AF system can be conventionally calculated as an integral evaluation value that is obtainable by integrating line peak values (obtained by peak-holding the focus signal along each horizontal line) in the vertical direction.
The integral evaluation value is stable and robust against instant noises due to effect of integration. Further, the integral evaluation value is excellent in sensitivity. Namely, the signal is sensitively variable in response to a very small focal shift. Thus, the integral evaluation value can be used to identify the in-focus direction.
Further, in the TV-AF system, there may be a problem that a point light source object cannot be focused. To solve the problem, as discussed in Japanese Patent Application Laid-Open No. 2006-189634, it is conventionally feasible to switch the integral evaluation value to be used in the control of the TV-AF system to a value obtainable by subtracting the present integral evaluation value from the maximum value of the integral evaluation value, in the vicinity of the in-focus point, if the presence of any point light source object is detected.
However, according to the integral evaluation value discussed in Japanese Patent Application Laid-Open No. 7-298120, a lens position corresponding to a large value may not coincide with the in-focus position in an image capturing operation of a point light source object. In general, if the target in a shooting operation is an ordinary object, the maximum point of the integral evaluation value coincides with the in-focus point as illustrated in FIG. 2.
On the other hand, if the target is a point light source object, the shooting operation is somewhat different. If the captured image of a point light source object is presently in focus as illustrated in FIG. 3A, the number of scanning horizontal lines available to capture the object is small although the focus signal has a very large line peak value. Therefore, the integral evaluation value does not increase so much.
However, if the captured image of a point light source object is presently out of focus as illustrated in FIG. 3B, the number of scanning horizontal lines available to capture the object is large although the focus signal has relatively smaller line peak values. Therefore, the integral evaluation value increases significantly.
From the reasons described above, the maximum point of the integral evaluation value does not coincide with the in-focus point as illustrated in FIG. 3C. Accordingly, if the system performs focus adjustment in such a way as to increase the integral evaluation value monotonously, an out-of-focus point may be erroneously detected as an in-focus point and a point light source object may not be focused adequately.
To solve the above-described problem, the technique discussed in Japanese Patent Application Laid-Open No. 2006-189634 is conventionally available. However, the conventional technique discussed in this patent literature is only useful to detect a minimum value that corresponds to the in-focus point illustrated in FIG. 3C. Therefore, the conventional technique requires a special control that is different from that of the TV-AF system, which identifies the maximum point of the integral evaluation value as the in-focus point.
Further, the minimum point (i.e., the in-focus point) may not clearly appear if shooting conditions for a target point light source object are inappropriate. In this case, it is difficult to focus on the target point light source object.